Fresenius J Anal Chern (1994) 349:477-478 - © Springer-Verlag 1994

477

Differential pulse polarographic determinationof trace levels of iron(ill)by using the catalytic current

Bharathibai J. Basu ', D. K. Padma ', S. R. Rajagopalan '

1 Materials Science Division, National Aerospace Laboratories,Bangalore 560017, India2 Department of Inorganic and Physical Chemistry,Indian Institute of Science, Bangalore 560003, India

Recei ved: 3 December 1993/Revised: 27 January 1994

Abstract. Trace of iron (III) are determined by differential pulsepolarography in a medium of sodium hydroxide and sodium bro-mate using the catalytic current. Various cations do not interfere.The relative standard deviation is 2%.

Introduction

Catalytic regeneration of the analyte on the electrode surface inthe presence of oxidizing agents can give rise to enhanced sensi-tivity for some elements in polarography. Thus catalytic currentshave been observed for iron (III) in the presence of hydrogenperoxide [1-5] and bromate [6-9]. In the former case, thewave shapes were not satisfactory due to a large maximum. Za-rebski [8] has reported a polarographic method for the determi-nation of iron using alkaline triethanolarninebromate as support-ing electrolyte. Later Ferri and Buldini [9] employed the samemedium to achieve a 60 times enhancement in sensitivity. Wefound that the sensitivity of the catalytic current could be furtherimproved in a medium of sodium hydroxide and sodium bro-mate. The method is ideally suited for the determination of tracelevels of impurity iron in alkaline solutions. The characterizationof .the electrode process was done using Birke's diagnostic cri-tena.

Experimental

The polarography with the cell assembly had been describedearlier [10]. The rate of flow of mercury was 1.862 mg S-I. ForDPP measurements, the pulse duration was 0.04 s and the pulseamplitude was 50 mV. The droptime was mechanically con-trolled. All reagents were of analytical reagent grade. A1000 ppm tron(III) stock solution was prepared by dissolvinghigh-purity iron powder in a mixture of hydrochloric and nitricacid and diluting to volume with distilled water.

Procedure. Transfer an aliquot of the sample into a polaro-graphic cell contairnng 4.0 ml 1 mollL sodium hydroxide and4.0 ml 1 mol/L sodium bromate and dilute to 20 mL. Deaeratethe solution by bubbling pure nitrogen for 10 min and recordthe polarogram from -0.80 V to -1.40 V vs. SCE. Repeat theprocedure after a standard addition of Fe(III).

Results and discussion

It was observed that the peak current of Fe(III) in a supportingelectrolyte (S.E.) of sodium hydroxide was enhanced by the ad-

Correspondence to: S. R. Rajagopalan

dition of sodium bromate. The enhancement factors were 75 and120 for 0.1 mollL and 0.2 mollL sodium bromate, respectively.The peak potential was at -1.135 V vs. SCE. The effect ofvariation of S.E. on the peak current was investigated. When thesodium hydroxide concentration was varied from 0.1 mollL to0.5 mol/L in a S.E. containing 0.1 mol/L sodium bromate, thepeak current remained almost constant. The peak potentialshifted slightly into the negative direction. The dependence ofthe peak current on the concentration of sodium bromate wasstudied: it increased with increasing sodium bromate concen-tration in a non-linear manner, The peak current was found tobe a linear function of the square root of the bromate concen-tration. This is characteristic for catalytic electrode processes.

Catalytic currents are known to have a higher temperaturecoefficient than diffusion controlled currents. The peak currentswere measured at various temperatures in the range of 25 toSO°c. It was found that i, initially increased with temperaturefrom 25 to 30°C, remained almost constant from 30 to 40°Cand again increased sharply in the range of 40 to 50°C. Thetemperature coefficient at 25°C was approximately 10%;oC.Thus a 67% increase in sensitivity could be obtained by re-cording the polarograms at 35°C.

The DPP diagnostic criteria of Birke et al. [11, 12] wereused to characterize the mechanism of the electrode 'process.The ratio of anodic to cathodic peak current was 1.0 and thedi~er.en~e in peak potentials was equal to the pulse amplitude.ThIS mdicated that the electron transfer step was reversible. Thehalf-peak width value at LiE = 50 mV was found to be 99 mVwhich was nearly equal to the theoretical value (98.2 mY) corre~sponding to a reversible electrode process with n = 1 and LiE =50 mV. A logarithmic analysis of the d.c. polarogram of iron(III)in this medium also showed that the electrode process was re-versible. The enhancement of the current in the presence of bro-mate confrrmed the catalytic nature, Hence, the electrode pro-cess is an EC (catalytic) mechanism with reversible electrontransfer.

The catalytic rate constant (k) was calculated using the ex-pression iJid = (n (j k cz)ll2, where i.li; is the enhancement inDP peak current in the presence of an oxidant, c, is the concen-tration of the oxidant and (j is the pulse duration [12]. Substitut-ing the experimental values, the second order rate constant kwas found to be 5.76xI05 S-1 L mol:".. Themolecular solubility of ferric hydroxide is not negligibleIII alkaline solutions because iron(III) may form soluble com-plexes of the type Fe(OH);. Hence the probable mechanism forthe catalytic reaction can be expressed as follows:

FeIU(O ).• + e ~ FelI(OH), + OH- , (1)

6 FeTI(OH), + BrO, + 3 H20 ~ 6 Fem(OH).• + Be . (2)

The sensitivity for the determination of iron(III) by the cata-lytic current in a S.E. of 0.2 mollL sodium hydroxide and0.2 ~~l/L sodium bromate was 24.0 nA L !!g-l under the DPPconditions, t = 2.0 sand LiE = 50 mV. The enhancement insensitivity was about 120 times that of the diffusion current.This enhancement factor was two times higher than that reportedearlier by Ferri and Buldini [9]. The linearity of the calibrationgraph was checked. The catalytic current was proportional tothe rron(III) concentration in the range 0.001 to 0.5 ppm in aS.E. of 0.2 mollL sodium hydroxide and 0.2 mollL sodiumbromate. A least square fit of the data was done by y-resi-dual minimization. The slope of the calibration was 8 nA L !!g-l(t = 0.5 s; LiE = 50 mV) and the correlation coefficient was0.9994.. Furthermore, the effect of cations on the catalytic current ofIron(III) was studied. Cations like Cu(ll), Pb(lI), Cd(II), Ni(ll),Co(II), Cr(lll), Mn(lI) and Sn(lI) did not interfere when present

478

in about 100-fold weight ratio to Fe(II1). 1000-fold amounts ofAl(III), Zn(II) and Mg(II) did not interfere, too. The catalyticcurrent was not affected by 10-fold amounts of Cr(VI), buthigher concentrations interfered by broadening the peak.

Since the method is highly sensitive as well as selective, itcan be used for the determination of trace levels of iron in vari-ous samples, such as natural fresh water samples and reagentchemicals like sodium hydroxide and potassium hydroxide. Itcan be applied for the evaluation of iron in any alkaline solution.No sample pretreatment is required. This is one of the advan-tages of this procedure. The standard addition method was usedto evaluate the concentration of iron in the samples. The pre-cision was estimated by six replicate measurements of 0.1 ppmiron(II1) in a synthetic sample solution. The relative standarddeviation was found to be 2%. Thus the method can serve as analternative to the more commonly used photometric methods forthe determination of trace levels of iron in aqueous samples andalkaline solutions.

References

1. Pospisil Z (1953) CoIl Czech Chern Commun 18 :3372. Kolthoff 1M, Parry EP (1951) J Am Chern Soc 73: 37283. Galus Z (1976) Fundamentals of electrochemical analysis.

Ellis Horwood, p 3284. Koryta J (1954) ColI Czech Chern Commun 19: 6665. Milyavsky YuS, Sinyakova SI (1968) Zh Anal Khim

23: 11836. Murali MK, Brahmaji Rao S (1980) Talanta 27: 9057. Hasebe K, Yamamoto Y, Ohzeki K, Kambara T (1986) Fre-

senius Z Anal Chern 323 :4648. Zarebski J (1977) Chemia Anal 22: 10499. Ferri D, Buldini PL (1981) Anal Chim Acta 126:247

10. Bharathibai J Basu, Rajagopalan SR (1989) Indian J Tech-nol 27: 232

11. Birke RL, Kim MH, Strassfeld M (1981) Anal Chern53:852

12. Kim MH, Birke RL (1983) Anal Chern 55: 522

Fresenius J Anal Chern (1994) 349:478-481 - © Springer-Verlag 1994

Extraction-spectrophotometric determinationof nickel as Ni-(PAR)2-(CTAB)2 complexin polymetallic sea-bed nodules and steels

S. N. BhadanP, Madhu Tewari ', Archana Agrawal",K. Chandra Sekhar '

1 Department of Chemistry, Ranchi University, Ranchi, India2 Analytical Chemistry Division, National MetallurgicalLaboratory, Jamshedpur 831 007, Bihar State, India

Received: 4 October 1993/Revised: 11 January 1994

Abstract. A red, water-soluble complex of nickel with PAR canbe extracted into chloroform with CTAB at pH 7.0. The systemobeys Beer's law upto 0.5 ug/ml with a molar absorptivity of45200 L . mol " . cm " at 540 nm. Job's method of continuousvariations revea!ed that the composition of the extracting speciesIS 1: 2 :2 for nickel: PAR: CTAB. Based on this extraction, ahighly sensitive and selective spectrophotometric method for thedetermination of nickel in polymetallic sea-bed nodules and insteels, after prior separation of iron and manganese, was devel-oped. The standard deviation was 0.04-0.127 flg for 5-25 pgof nickel.

Introduction

Deep-sea nodules are concretions that are roughly spherical inshape and vary in size from a few millimeters to several centi-meters. They are found on the ocean floor at depths anywherefrom several hundred to several thousand meters and their abun-dance and composition vary considerably from one location toanother [1].

Chemical analysis of sea nodules has evoked constant scien-tific interest since their discovery. Atomic absorption spec-

Correspondence to,' K. C. Sekhar

trometry (AAS) is regularly used in our laboratory for the deter-mination of Cu, Co and Ni in such samples [2]. In view of theircomplex matrix, interelement interferences play a crucial rolefor reliability of the analytical results. The problem is furthercomplicated by the absence of a sufficient number of referencesea nodule standards [3].

Computational identification and elimination of interelemen-tal interference has received popular attention and differenttheoretical models have been proposed [3-5]. But in all thesemodels the total effect of interferents is approximated as the sumof the effects of individual interferents on the analyte signal.This is not always accurate.

In spite of the widespread use of AAS, spectrophotometryremains widely employed in routine analysis because of its sim-plicity. In recent years, ion-pair metal complexes containing theanalyte, an organic chromophore reagent and a surfactant havebeen used widely and effectively for the spectrophotometric de-termination of trace amounts of metal ions [6].

Nickel combines with 4-(2-pyridyl azo)resorcinol (PAR) inthe ratio 1 :2 to form a water-soluble, red coloured complex withan absorbance maximum at 520 nm (PH 3.3, e = 37 200) and at496 nm (pH 8, e = 79 400) [7]. An attempt has been made inour present. study to selectively extract this Ni-PAR complexWIth cetyltrirnethyl ammonium bromide (CTAB) into chloro-form. A method is proposed for the determination of nickel inpolymetallic sea-bed nodules and steels and the results are pre-sented.

Experimental

Apparatus

Absorbance measurements were made with a Shimadzu 2100 Sdouble-beam UV-VIS spectrophotometer. pH-measurementswere made on a Unitech UI-11P pH meter.

Reagents

All reagents were of analytical reagent grade unless specifiedotherwise.